A Complex Life - Introduction to Coordination Chemistry - page 232

it should be remembered that s-block elements, in particular, are also important; for example
apart from its structural role in bones, calcium has a key role in triggering muscle action,
and magnesium appears in several transferase , phosphohydrase and polymerase enzymes.
While metals are met usually in trace amounts, balance is important in biosystems - both a
deficiency and an excess of metals can cause disease. Fortunately, a balanced diet provides
sufficient amounts of essential metals.
Vanadium in extremely small amounts is a nutritional requirement for many organisms,
including higher animals. Some marine organisms (tunicates) accumulate vanadium,
whereas some lichens and fungi contain vanadium in the active site of some enzymes.
Vanadium appears in one form of the important enzyme nitrogenase (which converts
dinitrogen to ammonium ion), and as vanadium(V) in haloperoxidase (which coordinates
hydrogen peroxide and then oxidizes halides).
Chromium , as Cr(III), is an essential trace element in mammals, and participates in glucose
and lipid metabolism. Chromium may help in maintaining normal glucose tolerance by
regulating insulin action. There is no evidence of significant chromium(III) toxicity,
but chromium(VI) and chromium(V) are toxic (mutagenic and carcinogenic). Cr(V) is
believed to damage DNA by promoting cleavage and DNA-protein cross-linking.
Manganese is an essential element, and appears in a wide range of organisms, including
humans. It occurs in important enzymes and processes, such as in photosynthesis in
plants. It can have a redox role, using particularly its Mn(II) and Mn(III) oxidation
states, as in superoxide dismutase , whose role is to destroy undesirable superoxide ion
through conversion to oxygen and peroxide ion.
Iron is truly ubiquitous in living systems, being found in all life forms from bacteria through
to humans. It is at the active centre of molecules responsible for oxygen transport, for
electron transport and in a vast range of enzymes. In the human body, hemoglobin and
myoglobin are vital components for oxygen transport, and represent 65% and 6% re-
spectively of all iron in the body. There is a wide range of other human iron-containing
proteins, however, with mainly redox roles, making use of accessible oxidation states,
notably Fe(II) and Fe(III). The iron proteins tend to be classified as either hemes (fea-
turing cyclic aromatic nitrogen ligands) or nonhemes. Iron is so significant that a special
iron storage protein, ferritin , is used to allow rapid access to iron when required.
Cobalt is an essential element in small amounts. Cobalt complexes of macrocyclic nitrogen-
donor ligands are found in many organisms, including humans, who contain about 5 mg
of these cobalamins. Vitamin B 12 is a cobalamin coenzyme required in humans for its
key role in promoting several molecular transformations. Recently, a range of enzymes
containing inert cobalt(III) has been discovered, distinguished by their unusual low
coordination number, which provides labile reaction sites despite the inherent inertness
of this ion.
Nickel is an essential element in small amounts, is a component of the important enzymes
urease , carbon monoxide dehydrogenase , hydrogenase and methyl-S-coenzyme M re-
ductase , and lies at their active sites. Nickel can exist under physiological conditions in
oxidation states I, II and III, but the higher two seem most relevant.
Copper occurs in almost all life forms and it plays a role at the active site of a large number
of enzymes. Copper is the third most abundant transition metal in the human body
after iron and zinc. Enzymes of copper include superoxide dismutase , tyrosinase , nitrite
reductase and cytochrome c oxidase . Most copper proteins and enzymes have roles as
electron transfer agents and in redox reactions, as Cu(II) and Cu(I) are accessible.